Fasteners such as bolts and nuts may tend to back or reverse out over time due to temperature changes, vibration, rotation, etc. In addition, fasteners may be tampered with by people attempting to unhook or open an item having fasteners. In some of these situations, it is helpful to have a fastener that cannot back or reverse out. Castle nuts are one type of nut that resists backing out. The castle nuts have slots that engage a cotter pin inserted through a bolt so that the nut cannot rotate on the bolt. Other nuts may have threads that use a plastic O-ring to create an interference fit with a bolt and prevent reversing out.